Blender’s main internal functionality (including GUI and editors) has been developed in C language. C++ has been used for writing game-engine, physics, audio functionality and render engine.
There is also a Python API for Blender with a very good documentation. This API can be used for building addons, and can also be used as a powerful tool for simplifying certain tasks by those who are familiar with Python.
Links : git.blender.org, Reference/FAQ – Blender Developer Wiki, Blender Python API Documentation
Random thought modules. 